Location Determination System Having Rotating Identifiers for Distinguishing Transmitters

ABSTRACT

The present disclosure relates to a location determination system that includes acoustic transmitting devices and mobile devices. A mobile device receives acoustic signals (e.g., ultrasound signals) from an acoustic transmitting device. The mobile device must be able to identify the acoustic transmitting device with particularity when receiving the acoustic signals. It is more efficient to use a short identifier, for example 6 bits, for this purpose because it requires less power to transmit and decode. By utilizing the rotating identifier system described herein, more acoustic transmitting devices can be distinguished than using a time-invariant identifier would otherwise provide, and the transmissions are more secure and therefore less vulnerable to free-riders.

FIELD

The present disclosure relates generally to real-time location systemsand more particularly to determining the location of an object or personwithin a real-time locating system.

BACKGROUND

Modern businesses and organizations face a common challenge in trackingthe location of important resources in a building or campus environment.Such resources include key personnel, critical pieces of equipment,vital records, or other useful devices. These resources often relocatethroughout the day according to organizational needs, and locating theseimportant resources can prove difficult and time consuming. In order toavoid the inherent productivity loss in devoting time and energy towardsmanually locating these resources, it is desirable to develop anapproach that tracks, catalogues, and reports the location of theseimportant resources in real-time.

SUMMARY

In an embodiment of the present disclosure, a computer-implementedmethod is described that includes receiving a modulated acoustic signalby a mobile device in a real-time location system in an environment, themodulated acoustic signal having been transmitted by an acoustictransmitting device that is located in the environment. Thecomputer-implemented method also includes detecting, by the mobiledevice, the modulated acoustic signal to determine an identifier in useat a time period, wherein the detected identifier is a member of arotating set of identifiers with each identifier in the set ofidentifiers having the same fixed bit length. The computer-implementedmethod also includes determining the location of the mobile device basedon the detected identifier, the time period, and additionally for largeinstallations, the RF access point to which the mobile device iscommunicatively coupled.

In a further embodiment of the present disclosure, acomputer-implemented method is disclosed that includes determining anidentifier of an acoustic transmitting device by a real-time locationsystem in an environment, wherein each of the identifiers is unique to aparticular acoustic transmitting device for a particular time period,and each of the identifiers has a same fixed bit length. Thecomputer-implemented method also includes modulating, by the acoustictransmitting device, an acoustic signal with the particular identifierto output a modulated acoustic signal during the particular time periodof validity. The computer-implemented method also includes transmitting,by the acoustic transmitting device, the modulated acoustic signalincluding the identifier to a mobile device located within theenvironment.

In an embodiment of the present disclosure, a computer-implementedsystem is described that includes an acoustic transmitting device thatis configured to use a rotating set of identifiers in a real-timelocation system in an environment, wherein each of the identifiers inthe rotating sequence of identifiers has a same fixed bit length. Theacoustic transmitting device is further configured to modulate anacoustic signal with the identifiers at the particular time period tooutput a modulated acoustic signal and transmit the modulated acousticsignal to a mobile device located within the environment during theparticular time period of validity. The computer-implemented systemfurther includes the mobile device that is configured to receive themodulated acoustic signal, to detect the identifier in the modulatedacoustic signal, to determine the location of the mobile device based onthe detected identifier, the time period, and additionally for largeinstallations, a rough indication of the neighborhood in which themobile device is located. The rough indication may be determined by themobile device based at least in part on a global positioning service(GPS), one or more Wi-Fi signals, one or more Bluetooth signals, one ormore cellular signals, one or more positioning sensors implementedwithin the mobile device (e.g. inertial measurement unit(s),gyroscope(s), accelerometer(s), magnetometer(s), etc.), one or morepressure sensors implemented within the mobile device, one or morecameras implemented within the mobile device, and/or other suitablemanner.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and form a partof the specification, illustrate embodiments of the present disclosureand, together with the description, further explain the principles ofthe disclosure and enable a person skilled in the pertinent arts to makeand use the embodiments.

FIG. 1 illustrates a perspective representation of a real-time locationsystem according to example embodiments of the present disclosure.

FIG. 2 illustrates a block diagram of an exemplary rotating identifiersystem as used in a real-time location system according to exampleembodiments of the present disclosure.

FIG. 3 illustrates a flowchart diagram of a method of identifying anacoustic transmitting device based on a modulated acoustic signal asused in a real-time location system according to example embodiments ofthe present disclosure.

FIG. 4 illustrates an example computing system according to exampleaspects of the present disclosure.

The present disclosure will be described with reference to theaccompanying drawings. In the drawings, like reference numbers indicateidentical or functionally similar elements. Additionally, the left-mostdigit of a reference number identifies the drawing in which thereference number first appears.

DETAILED DESCRIPTION

The following Detailed Description refers to accompanying drawings toillustrate exemplary embodiments consistent with the disclosure.References in the Detailed Description to “one exemplary embodiment,”“an exemplary embodiment,” “an example exemplary embodiment,” etc.,indicate that the exemplary embodiment described may include aparticular feature, structure, or characteristic, but every exemplaryembodiment does not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases do not necessarilyrefer to the same exemplary embodiment. Further, when the disclosuredescribes a particular feature, structure, or characteristic inconnection with an exemplary embodiment, those skilled in the relevantarts will know how to affect such feature, structure, or characteristicin connection with other exemplary embodiments, whether or notexplicitly described.

The exemplary embodiments described herein provide illustrative examplesand are not limiting. Other exemplary embodiments are possible, andmodifications may be made to the exemplary embodiments within the spiritand scope of the disclosure. Therefore, the Detailed Description doesnot limit the disclosure. Rather, only the below claims and theirequivalents define the scope of the disclosure.

Hardware (e.g., circuits), firmware, software, or any combinationthereof may be used to achieve the embodiments. Embodiments may also beimplemented as instructions stored on a machine-readable medium and readand executed by one or more processors. A machine-readable mediumincludes any mechanism for storing or transmitting information in a formreadable by a machine (e.g., a computing device). For example, in someembodiments a machine-readable medium includes read-only memory (ROM);random-access memory (RAM); magnetic disk storage media; optical storagemedia; flash memory devices; electrical, optical, acoustical or otherforms of propagated signals (e.g., carrier waves, infrared signals,digital signals, etc.), and others. Further, firmware, software,routines, instructions may be described herein as performing certainactions. However, it should be appreciated that such descriptions aremerely for convenience and that the actions result from computingdevices, processors, controllers, or other devices executing thefirmware, software, routines, and/or instructions.

Any reference to the term “module” shall be understood to include atleast one of software, firmware, and hardware (such as one or morecircuit, microchip, or device, or any combination thereof), and anycombination thereof. In addition, those skilled in relevant arts willunderstand that each module may include one, or more than one, componentwithin an actual device, and each component that forms a part of thedescribed module may function either cooperatively or independently ofany other component forming a part of the module. Conversely, multiplemodules described herein may represent a single component within anactual device. Further, components within a module may be in a singledevice or distributed among multiple devices in a wired or wirelessmanner.

The following Detailed Description of the exemplary embodiments willfully reveal the general nature of the disclosure so that others can, byapplying knowledge of those skilled in relevant arts, readily modifyand/or customize for various applications such exemplary embodiments,without undue experimentation and without departing from the spirit andscope of the disclosure. Therefore, such modifications fall within themeaning and plurality of equivalents of the exemplary embodiments basedupon the teaching and guidance presented herein. Here, the phraseologyor terminology serves the purpose of description, not limitation, suchthat the terminology or phraseology of the present specification shouldbe interpreted by those skilled in relevant arts in light of theteachings herein.

The present disclosure provides a real-time location system that tracksaccurate location information of objects and persons. Real-time locationsystems operate with different levels of accuracy depending on availablesystem infrastructure. In some embodiments, room-level accuracy oflocation information is sufficient whereas other use cases require 1D,2D or even 3D resolution of position within a room. In some embodiments,a system provides three-dimensional location information about personsor equipment in real-time. A real-time location system can include anetwork of acoustic transmitting devices attached to interior surfacesin an environment and mobile devices attached to moveable objects orpeople. The mobile devices receive signals from the acoustictransmitting devices to determine descriptive location information orthree-dimensional locational coordinates within the environment.Acoustics, e.g., ultrasound, is well-suited for this purpose because ittravels slower than radio waves and generally goes unnoticed by humans.Acoustic waves also attenuate more rapidly and do not penetrate walls,which avoids signal interference between rooms. If the location of theacoustic transmitting device is known, then the location of the mobiledevice is in the near vicinity of the location of the particularacoustic transmitting device whose signals have been received. Forexample, if a particular acoustic transmitting device has been installedin a closed room and a mobile device receives the acoustic signal fromthe particular acoustic transmitting device, then the mobile device islocated within the closed room. Acoustic signals are also easier toprocess when measuring a relatively short distance, as exists betweenthe acoustic transmitting devices and the mobile devices describedherein.

However, this approach presents unique challenges. The mobile devicesneed to have a particularized way of identifying the acoustictransmitting device that is the source of a modulated acoustic signal(modulated to carry information). Thus, the system assigns each acoustictransmitting device a unique identifier comprising a fixed number ofbits. Limiting the identifiers to be as short as possible, e.g., 6 bits,provides efficiency advantages because a shorter identifier requiresless power to be transmitted and less processing to detect. However,this approach limits the number of unique identifiers available. Forinstance, if an embodiment uses 6 bits for the identifiers, only 64(2{circumflex over ( )}6) unique identifiers exist. In an exemplaryinstallation, the number of acoustic transmitting devices may be muchhigher than 64, and thus, mobile devices need an alternative means orancillary means of identifying an acoustic transmitting device.

Therefore, a need exists for real-time location systems utilizing therotating identifier system described herein in order to enhance securityby preventing intrusive, malicious, or careless software or hardwaredevices from spoofing the system or exploiting, disturbing, or takingadvantage of the system infrastructure.

Real-time Location Systems

Real-time location systems have been developed using various wirelessprotocols with perhaps the best known system being the globalpositioning system (GPS). While such location systems provide horizontallocation accuracies on the order of approximately 8 meters, thesesystems do not solve all location-tracking scenarios. For example, manyscenarios require location accuracies of less than 0.3 meters. Otherscenarios require the ability to distinguish between floors in ahigh-rise building. Still other scenarios necessitate contextuallocation information, such as room-based information in an officebuilding.

Embodiments of the present disclosure provide solutions to theseaugmented location requirements. Acoustic signals may be used todetermine the location of mobile devices, attached to persons orobjects, by transmitting acoustic signals from acoustic transmittingdevices, which may be fixed to the walls or ceilings of a building. In afurther exemplary embodiment, the acoustic signals include identifiers(including encoded identifiers) assigned to their respective acoustictransmitting device. The mobile devices detect or decode the identifyinginformation in the acoustic signals. If the location of the acoustictransmitting device is known, then the location of the mobile device isin the vicinity of the location of the particular acoustic transmittingdevice whose signals have been received. For example, if a particularacoustic transmitting device has been installed in a closed room and amobile device receives the acoustic signal from the particular acoustictransmitting device, then the mobile device is located within the closedroom. Therefore, if each of the acoustic transmitting devices emits aunique identifying signal and the location of each acoustic transmittingdevice is known, then the location of a mobile device may be determinedupon its receipt of an acoustic signal from a particular acoustictransmitting device and its associated identity. Similarly, if a mobiledevice receives acoustic signals from each of two separatelyidentifiable acoustic transmitting devices, then the mobile device islocated in the vicinity of both of the two separately identifiableacoustic transmitting devices. For example, were the two separatelyidentifiable acoustic transmitting devices placed at the two oppositeends of a corridor, then a mobile device situated in the corridor wouldlikely receive signals from the two separately identifiable acoustictransmitting devices, and the location of the mobile device would beestablished. In other embodiments, more precise locations of a mobiledevice may be established. For example, arrival times of acousticsignals at a mobile device can be used to find the location of themobile device in the environment, using standard geometric calculations.

In an embodiment, the acoustic signals may also include data associatedwith the environment near the acoustic transmitting device such as oneor more rooms, spaces, structures, buildings, regions, etc. in which theacoustic transmitting device resides. More particularly, suchenvironmental data can include specific details associated with theenvironment. For instance, the environmental data can indicate therespective room, building, campus, area, etc. where the acoustictransmitting device is located. The environmental data can furtherinclude data specifying an organization, configuration, or hierarchy ofthe environment in which the acoustic transmitting device is located.For instance, such environmental data can include data specifying arelationship between a particular room and a particular building (e.g.,a location of the room within the building).

The environmental data can further include dimensional data associatedwith the environment. For instance, dimensional data can include thedimensions of one or more reflective surfaces (e.g. walls, ceilings,floors, objects, furniture, etc.) within a room in which a transmittingdevice is located. The dimensional data can further include dataindicative of the normal direction of the reflective surfaces. Theenvironmental data can further include data indicative of the acousticattenuation of such reflective surfaces. The environmental data canfurther include data indicative of the relative locations of theacoustic transmitting devices within a particular room, building, area,etc. More particularly, such environmental data can include anidentifier of a surface (e.g. wall, floor, ceiling, etc. of a room) onwhich a transmitting device is located and/or data indicative of alocation and/or orientation of the transmitting device with respect tothe surface. The environmental data can further include atmospheric dataindicative of the speed of sound, temperature, pressure, humidity, etc.within the environment. In certain embodiments, since the environmentaldata changes over time, the environmental data updates frequently toreflect current environmental conditions.

The acoustic transmitting devices of the real-time location system canbe configured to periodically transmit acoustic signals (or othersuitable signals, such as radio frequency signals) to be received bymobile devices located within broadcast range of the transmittingdevices. In some implementations, the acoustic signals can be ultrasonicsignals having a frequency of about 20 kHz or greater. In a particularembodiment of the present disclosure, the acoustic signals can beultrasonic signals having a frequency of about 20 kHz. In anotherparticular embodiment of the present disclosure, the acoustic signalscan be ultrasonic signals having a frequency of about 40 kHz. As usedherein, the term “about,” when used in reference to a numerical value,refers to within 30% of that value.

In this manner, a mobile device within the broadcast range of theacoustic transmitting devices picks up acoustic signals. The acousticsignals can be signals propagating directly from the acoustictransmitting devices to the mobile devices (referred to herein as“direct signals”) and/or signals reflected by one or more reflectivesurfaces (referred to herein as “reflected signals”). The reflectivesurfaces can act as acoustic mirrors capable of reflecting acousticsignals (with some attenuation) and can include walls, ceilings, floors,furniture, objects, etc. located within the environment. The preciselocation of a mobile device can be determined based at least in part onthe acoustic signals received from the acoustic transmitting devices. Insome implementations, contextual or descriptive location information maybe provided, e.g., room number or floor number in an office building.

A mobile device must appropriately identify the source of the acousticsignals being received, i.e., the identifier of the acoustictransmitting device that is transmitting the acoustic signals.Minimizing the digital size of the identifiers, e.g., to 6 bits,provides efficiency advantages because a shorter identifier (fewernumber of bits) requires less power to be transmitted and lessprocessing to detect. However, this approach limits the number of uniqueidentifiers available. For instance, if an embodiment of the identifiersuses 6 bits, only 64 (2{circumflex over ( )}6) unique identifiers areavailable. In an exemplary installation of a real-time location system,the number of acoustic transmitting devices may be much higher than 64,and thus, mobile devices need an alternative means of identifying, or anancillary means of identifying, an acoustic transmitting device.

To overcome the limitations of the fixed number of bits for anidentifier, each acoustic transmitting device transmits a modulatedacoustic signal carrying an identifier that changes (or rotates) overtime in a manner that is known to the relevant components in thereal-time location system. In this approach, a unique identifier is usedfor a particular time period, and then at the end of this time period, asecond (different) unique identifier is used for a subsequent timeperiod. Continuing in this manner, a sequence of identifiers is used byeach acoustic transmitting device, where each identifier in the sequenceis used for a pre-defined time period, and then replaced in the nexttime period with another identifier in the sequence. The pre-definedtime periods are known by components in the real-time location system,with these components being time synchronized to a universal time in thereal-time location system. In an embodiment, a mobile device receivesthe modulated acoustic signals from an acoustic transmitting device,detects the identifier in the modulated acoustic signals, and uses thecombination of the detected identifier together with the time periodwhen the detected identifier is used to determine the particularacoustic transmitting device that transmitted the modulated acousticsignals. By determining the particular acoustic transmitting device, thelocation of the mobile device that receives the modulated acousticsignals from the particular acoustic transmitting device is alsodetermined (i.e., in the immediate vicinity of the particular acoustictransmitting device.). In an alternate embodiment, the mobile devicepasses on the detected identifier to a remote processing server (e.g.,central server) via a wireless network connection, a Local-Area Network(LAN), a Wide-Area Network (WAN), or other communication network orprotocol. In this embodiment, the central server determines theparticular acoustic transmitting device whose acoustic signal wasreceived by the mobile device by processing the combination of thedetected identifier together with the time period when the detectedidentifier is used to make the determination of the particular acoustictransmitting device. Since the location of each acoustic transmittingdevice is known, the location of a mobile device may be ascertained byusing the combination of the detected identifier in combination with thetime period in which the detected identifier is in use. In certainembodiments, a lower limit on this time period may be determined to beabout 60 milliseconds. The 60 millisecond value is based on the maximumacoustic range of 20 meters, a maximum that is set by acousticattenuation such that signals beyond this range are below the noiselevel in a typical environment.

In scenarios where a real-time location system is desired to cover avery large environment, a rotating set of identifiers may not providethe uniqueness required across the entire environment. In other words,at any given time period, a particular identifier may be in use by morethan one acoustic transmitting device, when a large number (e.g.,hundreds) of acoustic transmitting devices are in use. To address suchsituations, the lack of uniqueness of the identifiers of acoustictransmitting devices in any given time period may be resolved by using arough indication of the neighborhood in which the particular acoustictransmitting device and mobile device are located. The rough indicationmay be determined by the mobile device based at least in part on aglobal positioning service (GPS), one or more Wi-Fi signals, one or moreBluetooth signals, one or more cellular signals, one or more positioningsensors implemented within the mobile device (e.g. inertial measurementunit(s), gyroscope(s), accelerometer(s), magnetometer(s), etc.), one ormore pressure sensors implemented within the mobile device, one or morecameras implemented within the mobile device, and/or other suitablemanner. Consequently, whether the mobile device itself, or a centralserver, performs the processing to determine the location of the mobiledevice, the mobile device/central server can use the combination ofthree pieces of information: the detected identifier of an acoustictransmitting device whose acoustic signals have been received, the timeperiod when the detected identifier was in use, and the RF access pointin use by the mobile device to uniquely determine the acoustictransmitting device (and therefore the location of mobile device).

The systems and methods of the present disclosure can be used in anumber of applications including, for example, location tracking, workflow, mobile equipment tracking, safety and compliance, mobile equipmentmanagement, staff location determination, or other suitableapplications. One example field of use is within the health careindustry. In an embodiment, a hospital implements a real-time locationtracking system of the present disclosure to provide patient tracking,patient flow, asset management, environmental monitoring, etc.

FIG. 1 is a perspective representation of location determination system100. Location determination system 100 can be a real-time locationsystem in an environment that determines the location of a moveableobject or person. Placed within environment 102, location determinationsystem 100 can include acoustic transmitting device 104, mobile device106, remote processing server 108, and modulated acoustic signals 110.These components cooperate to provide a location system capable ofestimating a location of mobile device 106 within environment 102. Incertain embodiments, location information may be three-dimensionallocation information. In some embodiments, location determination system100 includes more than one instance of acoustic transmitting device 104installed throughout a building or series of rooms, and more than oneinstance of mobile device 106 attached to, or incorporated into/onto,people, machines, animals, vehicles, robots, stock, equipment, or otherobjects. Environment 102 can consist of rooms in a building such as, forexample, a ward in a hospital, an office in an office building, or astorage space in a warehouse. More than one instance of environment 102can exist and include more than one instance of acoustic transmittingdevice 104. When several instances of environment 102 serve onelocation, building, or complex, these instances of environment 102 canbe incorporated into clusters, groups, or management entities. In analternate embodiment, environment 102 comprises a single room.

Acoustic transmitting device 104 includes an acoustical device, such asan ultrasonic transmitter, and processing logic to transmit modulatedacoustic signals 110. Modulated acoustic signals 110 transmitted byacoustic transmitting device 104 communicate an identifier that isunique to a specific instance of acoustic transmitting device 104 at aparticular time period. The identifier may be a number in apseudo-random sequence or be an identifier in a pre-defined table ofidentifiers. The provision of the identifier is made known to both theacoustic transmitting device 140 that transmits the identifier, as wellas the mobile device (or central server) that correlates the detectedidentifier with the particular acoustic transmitting device 140 thatuses the detected identifier at a particular time period. Theseparticular time periods are governed by a system-wide acoustictransmission schedule that requires that acoustic transmitting device104 and the mobile device are active at the same time, and inactive atthe same time. Such coordination maximizes the battery life of devicesby synchronization of the timing of transmitting/receiving theidentifiers necessary for location determination. In an embodiment,acoustic transmitting device 104 modulates/encodes the identifier on anultrasonic carrier having an ultrasonic frequency such as, for example,about 20 kHz, 40 kHz, or any other suitable ultrasonic frequencies. Asdescribed above, location determination system 100 can include more thanone instance of acoustic transmitting device 104, and each acoustictransmitting device 104 can be configured to transmit modulated acousticsignals 110 containing an identifier unique to each particular instanceof acoustic transmitting device 104. As noted above for scenarios wherea real-time location system is desired to cover a very largeenvironment, an identifier at any given time period may not be unique.In other words, at a particular time period, a particular identifier maybe in use by more than one acoustic transmitting device, when a largenumber (e.g., hundreds) of acoustic transmitting devices are in use. Toaddress such situations, a combination of three pieces of information:the detected identifier of an acoustic transmitting device whoseacoustic signals have been received, the time period when the detectedidentifier was in use, and the RF access point in use by the mobiledevice is used to uniquely determine the acoustic transmitting device(and therefore the location of the mobile device).

Mobile device 106 includes a microphone capable of receiving modulatedacoustic signals 110 from acoustic transmitting device 104, and mayinclude as well as a processing unit to sample, decode, detect andprocess any received modulated acoustic signals 110. Mobile device 106resides inside environment 102. Mobile device 106 may be a portabledevice, and may be attached to a person or item of equipment. In someembodiments, mobile device 106 includes devices such as, for example, acell phone, an acoustic transducer, an ultrasound transducer, anacoustic tag, an ultrasound tag, and/or any other suitable devices.

In some embodiments, mobile device 106 does not perform processing usingits own processing unit (or does not have a processing unit), butoffloads the processing to a remote computer such as remote processingserver 108 by transmitting relevant data to remote processing server 108using one or more appropriate communication channels, e.g., acoustic,ultrasound or radio frequencies. Mobile device 106 and/or acoustictransmitting device 104 include a wired or wireless transmitter, such asa radio transmitter, for transmitting information relevant to real-timelocation determination. In some embodiments, mobile device 106communicates with remote processing server 108 via radio frequencies, aLocal-Area Network (LAN), a Wide-Area Network (WAN), or othercommunication network or protocol.

Remote processing server 108 (e.g., central server) consists of one ormore servers processing real-time location data constituting, forexample, identities of mobile devices and acoustic transmitting devices,locations of acoustic transmitting devices, RF access points, etc.Remote processing server 108 employs standard communication modules(e.g., RF, wireline) to listen for, process, and respond to incomingsignals. Remote processing server 108 includes processes to performoperations and calculations and radio frequency modules to transmitsignals back to acoustic transmitting device 104 and mobile device 106.In an alternate embodiment, remote processing server 108 communicates toacoustic transmitting device 104 and mobile device 106 via a LAN, WAN,or other wireless/wired communication network.

Remote processing server 108 includes a database, which storesinformation about acoustic transmitting device 104 and mobile device 106and tracks locations in real-time. In an embodiment, the database may beany commercially available database management system such as MicrosoftAccess, Microsoft SQL server, an Oracle database, an IBM database, etc.The database maintains communicative connections to the processingelements via traditional networking infrastructure such as routers,switches, hubs, firewalls, etc. In an embodiment, the database may besituated in one computer workstation. Remote processing server 108implements a centralized storage area network, network-attached storage,redundant array of independent disks, and/or any other configuration ofstorage devices to supply sufficient storage capacity to archive thefull panoply of locational information. Sufficient storage alternativelyexists in any other physically attached magnetic storage, cloud storage,or any additional storage medium. In an embodiment, remote processingserver 108 deploys a commonly utilized hard-disk interface, such as ATA,SATA, SCSI, SAS, and/or fibre for interfacing with a storage medium.

Modulated acoustic signals 110 include an assemblage of signalstransmitted from acoustic transmitting devices 104 that propagate withinenvironment 102. In an embodiment, modulated acoustic signals 110 fallin the ultrasonic range, i.e., 20 kHz up to 1 MHz and beyond. Specificembodiments include modulated acoustic signals at 20 kHz, and at 40 kHz.Location determination system 100 modulates, encodes, identifies, anddetects/decodes modulated acoustic signals 110 in order to differentiateamong various signals and determine locations.

Modulated acoustic signals 110 can include data descriptive ofcharacteristics of the acoustic signals including, for example, a soundpressure level, signal coding type, signal identification, signaldirection normal, signal spatial distribution, signal period, and/orother suitable data. Modulated acoustic signals 110 can further includedata associated with an environment covered by location determinationsystem 100. Such environmental data can include a layout ororganizational hierarchy of the environment, identifying data of thelocation within the environment (e.g. room, area, space, region,building, etc.) in which acoustic transmitting device 104 sits,dimensional specifications of one or more reflective surfaces (e.g.walls, ceilings, floors, objects, etc.) within the environment (e.g.within a room, area, region, etc. in which acoustic transmitting device104 is located), data indicative of the relative location of acoustictransmitting device 104 within the environment, such as an identifier ofthe surface on which acoustic transmitting device 104 is located and/ora location and/or orientation of acoustic transmitting device 104 withrespect to the surface. The environmental data can further includeatmospheric data indicative of the speed of sound, temperature,pressure, humidity, etc. within the environment. In certain embodiments,because the environmental data changes over time, the environmental datamay be frequently updated to reflect current environmental conditions.

Modulated acoustic signals 110 include an identifier that is used byacoustic transmitting device 104 for a particular time period, beforechanging (rotating) to a new identifier for the subsequent time period.In an embodiment, mobile device 106 receives modulated acoustic signals110 and detects (detection can include decoding, demodulation or othersignal processing to recover the transmitted identifier) the identifierof acoustic transmitting device 104 that transmitted those modulatedacoustic signals 110. From the identifier, mobile device 106 maydetermine the identity of the specific acoustic transmitting device 104by knowledge of which acoustic transmitting devices 104 were using thatparticular identifier at the particular time period. If acoustictransmitting device 104 uses a pseudo-random number generator togenerate the identifier, then mobile device 106 would use the samepseudo-random number generator to “reverse” the process, and therebytranslate the detected identifier into the identity of the acoustictransmitting device 104 that transmitted the identifier. Similarly, ifacoustic transmitting device 104 uses a pre-defined table of identifiersfor use in the successive time periods, then mobile device 106 would usethe same pre-defined table of identifiers to “reverse” the process, andthereby translate the detected identifier into the identity of theacoustic transmitting device 104 that transmitted the identifier. In analternate embodiment, remote processing server 108 performs thetranslation processing. In this alternate embodiment, mobile device 106forwards the detected identifier to remote processing server 108 via awireless network connection, a Local-Area Network (LAN), a Wide-AreaNetwork (WAN), or other communication network or protocol. In thisembodiment, remote processing server 108 performs the processing anddetermines the identity of acoustic transmitting device 104. In largeinstallations, the RF access point to which the mobile device iscommunicatively coupled is additionally used for the determination ofthe identity of acoustic transmitting device 104.

Determining the Identity of an Acoustic Transmitting Device

As noted above with respect to efficiency, limiting the identifiers ofthe acoustic transmitting devices to be as short as possible, e.g., ap-digit identifier limited to p=6 bits, is advantageous because shorteridentifiers require less power to be transmitted and less processingresources to detect. However, shortening the identifiers in this fashionlimits the number of unique identifiers available in the system. Forinstance, if an embodiment uses 6 bits for the identifiers, only 64(2{circumflex over ( )}6) unique identifiers exist. This presents achallenge because in an exemplary installation, more than 64 instancesof acoustic transmitting device 104 may exist. It is desirable to use anidentifier system that retains the efficiency advantages of usingshorter identifiers while allowing more than 64 (or 2{circumflex over( )}x, where x is the number of bits configured in that embodiment)instances of acoustic transmitting device 104 to coexist. Furthermore,it is desirable that unauthorized people are not able to map out thereal-time location system infrastructure for purposes of offering aseparate location service while not having to foot the expense ofbuilding the real-time location system infrastructure. Thus, forexample, a group of acoustic transmitting devices 104 that use fixed(i.e., time-invariant) identifiers are vulnerable to third-partyexploitation. For example, a group of three transmitting devices 104that uses the time-invariant identifiers 5, 53 and 28 could be readilyascertained by a third party. By taking this readily ascertainedinformation, a third party could then take this mapping information andprovide a real-time location system without funding the cost of theunderlying infrastructure. Thus, in addition to needing more than 64 (or2{circumflex over ( )}x, where x is the number of bits configured inthat embodiment) instances of acoustic transmitting device 104 tocoexist, it is desirable that an approach be provided that thwarts anyattempt to map the underlying infrastructure by an unauthorized party.

In embodiments of the present disclosure, a rotational identifier systemis described that overcomes the challenges of short length identifiers.Instead of a time-invariant single short-length integer that serves asan identifier for a given acoustic transmitting device 104, embodimentsdescribe the use of changing (or rotating) short-length integers thatserve to identify an acoustic transmitting device 104 at successive timeperiods. For example, the identifier for acoustic transmitting device104 may be “7” for a time period. In a subsequent time period, the sameacoustic transmitting device 104 may identify itself as “63” for thissubsequent time period.

In further embodiments, acoustic transmitting device 104 may transmitthe p-digit identifier for a predetermined time period, and thentransmit a different p-digit identifier for a subsequent time period. Inan exemplary embodiment, acoustic transmitting device 104 may repeat thetransmission of the p-digit identifier every one second during each timeperiod, where a time period may be one minute. In various embodiments,the time period may be in the range of 3 seconds to 300 seconds. As oneof ordinary skill in the art would recognize, this range is merelyexemplary and not limiting. By repeatedly transmitting the p-digitidentifier during each time period, mobile devices 106 are provided withmultiple opportunities to receive the transmitted p-digit identifier. Toreduce the overlap of the transmissions of two or more acoustictransmitting devices 104, each acoustic transmission device 104 may beassigned a time slot of transmission that differs from its neighboringacoustic transmitting device(s). For example, in an exemplary identitytransmission scheme where each acoustic transmitting device 104 repeatsits identifier transmissions every second, each second may be divided upinto a number of time slots. In a particular example, a one secondtimeframe may be broken up into 15 time slots, where one acoustictransmitting device 104 may transmit its acoustic signals in the firsttime slot, while a neighboring acoustic transmitting device 104transmits its acoustic signals in the second time slot. As noted above,these acoustic signal transmissions are repeated in each subsequent onesecond timeframe within the time period during which the assignedidentifiers are valid. For the next time period, different identifiersare used for each acoustic transmitting device 104. For example, if eachtime period is one minute, acoustic transmitting devices 104 repeatedlytransmit their identifiers every one second in their assigned time slot,and that change (rotate) at the end of their one minute time periods tothe new set of identifiers. These time periods are known by componentsin the real-time location system, with these components being timesynchronized to a universal time in the real-time location system.

In an alternative embodiment, the identifier may be a sequence offixed-bit codes, where the code changes upon every transmit inaccordance with a pseudo random sequence. The pseudo random sequence isgenerated by a pseudo random generator using a seed as input. Todetermine the identity of the acoustic transmitting device 104, Themobile device would then have to receive several transmitted codesbefore the underlying pseudo random seed can be uniquely identified, andthereby associated with transmitter location based on the constraintsset by the time period and approximate location of the mobile device. Inthis embodiment, the identifier is then effectively the pseudo randomseed. This approach would be computationally difficult at start-up,since it requires finding a unique seed based only on the approximatelocation of the mobile device. However, for identification of subsequentacoustic transmitting devices, the process is easier since the secondlocation determined from acoustic positioning constrains the number ofseeds that need to be considered to be those of its nearest neighbors.

As the above discussion notes, the scalability challenge of short bitinteger identifiers may be overcome by using a rotation (sequence) ofshort bit integer identifiers, where the particular identifier during aparticular time period provides the required unique identification. Insome embodiments, security is also a requirement to be addressed.Security requirements typically generate a need that acoustictransmitting devices 104 conceal their transmission of their identitiessuch that only pre-configured equipment (such as mobile devices) may beable to properly detect the identity information. Absent such securitymeasures, unapproved equipment may improperly “free ride” on theinfrastructure provided by someone else. To address thesesecurity/free-riding concerns, an acoustic transmitting device 104 maytransmit a rotating sequence of identifiers that is randomly generatedfor that device. By randomly generated, it is meant that a third partythat observes the identifier at a particular timeframe cannot determinewhat the observed identifier would be from the same acoustictransmitting device 104 at a later point in time. Approaches to therandomly generated sequence include the generation of a pre-determinedsequence, the generation of a pseudorandom sequence, and a completelyrandom sequence. Both the pre-determined sequence and the completelyrandom sequence require that these sequences be shared with thetransmitting device and the receiving/detecting device. A pseudo-randomsequence that uses a seed for its generation would require that the seedbe shared with the transmitting device and the receiving/detectingdevice. Where a server is employed, the server may provide theprocessing required under either the generation or the detectingaspects, and the acoustic transmitting device 104 and/or mobile devices106 may then be more simply implemented.

One approach to an implementation of the above methodology is for aserver (e.g., remote processing server 108) to manage the acoustictransmitting devices 104 and mobile devices 106 in accordance with thedesired randomization identification procedure. For example, a server(e.g., remote processing server 108) determines a number of seeds thatare valid for a period of time that begins at a particular time instant.In such an embodiment, a server maintains a universal time and forwardsa set of seeds to the acoustic transmitting devices 104 and mobiledevices 106 that are to be used for a number of successive time periodst₁, t₂, t₃, t₄, t₅, t₆, etc. For each acoustic transmitting device 104,a respective identifier is determined using its respective seed and therespective identifier is valid for time period t₁. The respectiveidentifiers would be forwarded to the location processing component,i.e., the mobile devices 106 or remote processing server 108. When theperiod of time t₁ completes, a new period of time t₂ begins and the nextidentifier is determined for each acoustic transmitting device 104. Thissequential process continues for successive periods of time t₃, t₄, t₅,etc. In an embodiment, during a reset, a new set of seeds would beforwarded to the transmitting devices 104 and mobile devices 106 for usein determining the identifiers following the reset.

FIG. 2 depicts rotating identifier system 200, according to embodimentsof the present disclosure. Rotating identifier system 200 includesidentifier management center 210 and one or more instances of acoustictransmitting device 104, mobile device 106, and modulated acousticsignals 110 from FIG. 1. In an embodiment, rotating identifier system200 organizes one or more instances of acoustic transmitting device 104into groups or clusters. The groups or clusters may track footprints ofvarious RF access points, and thereby provide for additionalidentification of acoustic transmitting devices in large facilitieshaving hundreds of acoustic transmitting devices. Rotating identifiersystem 200 provides a method to synchronize identifiers among one ormore instance of acoustic transmitting device 104 and one or moreinstances of mobile device 106. Rotating identifier system 200 providesseveral advantages, including increased efficiency and enhancedsecurity.

For security, rotating identifier system 200 ensures that intrusive,malicious, or careless software or hardware devices or programs cannotexploit, disturb, take advantage of, or otherwise interact withidentifier management center 210 and the devices of locationdetermination system 100. Furthermore, rotating identifier system 200prevents outside parties from utilizing the infrastructure installed inan environment for location determination system 100 for alternativepurposes or aims. Because malicious entities or free-riders are notprivy to rotating identifier system 200 as described in this embodiment,rotating identifier system 200 remains safe from foreign interactions,interruptions, free-riders, or intrusions.

Identifier management center 210 includes random number generator 211,seed value 212, and pre-defined sequence table 213. Identifiermanagement center 210 stores and manages the rotating identifiers andtransmits requisite information to acoustic transmitting device 104 andmobile device 106. In an alternate embodiment, identifier managementcenter 210 exists in a distributed architecture across acoustictransmitting device 104 and mobile device 106. Identifier managementcenter 210 communicates to devices in location determination system 100using radio frequencies or through a standard wireless or wired protocol(e.g., Bluetooth, WLAN, etc.). Such communication can be encrypted usingindustry standard methods such as public/private key encryption.Identifier management center 210 tracks the identifiers that areassociated with each acoustic transmitting device 104, and valid atparticular time periods. Identifier management center 210 may furthertrack seeds for random number generators, random number generationalgorithms, timestamps associated with time periods, time slots fortransmission of identifiers, and relationships between entities inlocation determination system 100.

Random number generator 211 generates and/or stores and retrieves asequence of pseudorandom numbers for use by identifier management center210 in determining the identity of acoustic transmitting device 104.Each of the numbers is valid for a particular time period. As oneskilled in the art will understand, the generation of random numbers canbe accomplished numerous ways including, for example, pseudorandomnumber generation, linear congruential generation, multiplicativecongruential generation, etc.

In embodiments, seed value 212 may be used for the random numbergeneration, and would be known among acoustic transmitting device 104and mobile device 106. Thus, all members of rotating identifier system200 can possess coordinated, time-synchronized random number generationsystems. In an alternate embodiment, random number generator 211 residesin remove processing server 108 and is shared between rotatingidentifier system 200, acoustic transmitting device 104, and mobiledevice 106.

Pre-defined sequence table 213 stores a representation of the availablesequences of identifiers as derived from the number of bits devoted forthe acoustic identifier, where each identifier in the sequence is validfor a particular time period. Pre-defined sequence table 213 is apre-defined table storing sequences of identifiers. In an embodiment,pre-defined sequence table 213 stores the sequences of identifiers in adatabase table, which can be housed in any commercially availabledatabase management system such as Microsoft Access, Microsoft SQLserver, an Oracle database, an IBM database, etc. In an alternateembodiment, sequence table 213 is not actually stored but is a logicalconstruction based on the number of bits used and exists only in RAM orat runtime.

FIG. 3 illustrates a flowchart diagram of a method of identifyingacoustic transmitting device 104 based on modulated acoustic signals110. Identification method 300 includes the following steps: receivesignal (step 301), detect received identifier (step 302), determinevalid identifiers at relevant time period (step 303), compare detectedidentifier to valid identifiers at relevant time period (step 304),match found (decision point 305), and location of mobile devicedetermined (step 306).

In receive signal (step 301), mobile device 106 receives modulatedacoustic signals 110, in an embodiment, by using a microphone capable ofreceiving modulated acoustic signals 110. Modulated acoustic signalsinclude the identifier (possibly encoded identifier information) fromthe acoustic transmitting device 104 that transmitted modulated acousticsignals 110, which changes according to the period of time. For example,at time t₁, the identifier information would reflect one identifier fora particular acoustic transmitting device 104, but at time t₂, adifferent identifier would be reflected in the identifier information.

In detect received identifier (step 302), mobile device 106 detects theidentifier from the signals received in receive signal (step 301). In anembodiment, detect received identifier (step 302) occurs on mobiledevice 106 and its components. In alternate embodiments, a remotecomputer, such as remote processing server 108 detects the receivedsignals and performs any necessary processing to determine theidentifier.

In determine valid identifiers at relevant time period (step 303),identification method 300 retrieves the valid codes from identifiermanagement center 210. In an embodiment, the current time period iscompared to the time periods in time-period specific transmitteridentification tables 212 to determine the valid identifiers at thespecific time period. The valid identifiers represent the one or moreinstances of acoustic transmitting devices 104 in environment 102.

In compare detected identifier to valid identifiers at relevant timeperiod (step 304), identification method 300 compares the receivedidentifier (from detect received identifier step 302) to the valididentifiers (from determine valid identifiers at relevant time periodstep 303) in order to determine an identity of the instance of acoustictransmitting device 104 from which acoustic signals 110 were received.In an embodiment, compare detected identifier to valid identifiers atrelevant time period (step 304) looks up, retrieves, or otherwisedetermines an appropriate identifier for an acoustic transmitting device104 by comparing the detected identifier received in modulated acousticsignals 110 to the list, array, or alternative data structurerepresenting valid identifiers. The detected identifier can be comparedusing any appropriate means of comparison, e.g., SQL, comparisonoperators, etc., as one skilled in the arts would understand.

In match found (decision point 305), identification method 300 matchesthe detected identifier from modulated acoustic signals 110 sent fromacoustic transmitting device 104 to predetermined identifiers assignedfor use at particular time periods by the acoustic transmitting devicesin a location determination system. In location of mobile devicedetermined (step 306), the location of the mobile device 106 is based onthe determination of the identifier of acoustic transmitting device 104based on the detected identifier at the relevant time period of thelocation determination system. Location determination system 100 usesthis detected identifier to determine and report the location of mobiledevice 106.

FIG. 4 depicts an example location system 400 that can be used toimplement the methods and systems of the present disclosure. In someimplementations, location system 400 is a real-time location systemconfigured to determine the locations of persons and objects. Locationsystem 400 can be implemented using a client-server architecture thatincludes mobile device 106 that communicates with one or more remotecomputing devices, such as remote processing server 108. Location system400 can be implemented using other suitable architectures.

As shown, location system 400 can include a mobile device 106. In anembodiment, mobile device 106 is any suitable type of mobile computingdevice, such as a smartphone, tablet, cellular telephone, wearablecomputing device, or any other suitable mobile computing device. In someimplementations, mobile device 106 is a dedicated tag (e.g., passive oractive) or other device for use in the real-time location system. Mobiledevice 106 can include one or more processors 402 and one or more memorydevices 404.

One or more processors 402 can include any suitable processing device,such as a microprocessor, microcontroller, integrated circuit, logicdevice, one or more central processing units (CPUs), graphics processingunits (GPUs) dedicated to efficiently rendering images or performingother specialized calculations, and/or other processing devices, such asa system on a chip (SoC) or a SoC with an integrated RF transceiver. Oneor more memory devices 404 can include one or more computer-readablemedia, including, but not limited to, non-transitory computer-readablemedia, RAM, ROM, hard drives, flash memory, or other memory devices.

One or more memory devices 404 can store information accessible by oneor more processors 402, including instructions 406 executed by one ormore processors 402. For instance, one or more memory devices 404 canstore the instructions 406 for implementing one or more modulesconfigured to implement acoustic transmitting device 104, mobile device106, remote processing server 108, and/or other suitable instructions.

Each of acoustic transmitting device 104, mobile device 106, and remoteprocessing server 108 can include computer logic utilized to providedesired functionality. Thus, each of acoustic transmitting device 104,mobile device 106, remote processing server 108 can be implemented inhardware, application specific circuits, firmware and/or softwarecontrolling a general purpose processor. In one embodiment, each ofacoustic transmitting device 104, mobile device 106, remote processingserver 108 are program code files stored on a storage device, loadedinto memory and executed by a processor, or provided from computerprogram products, for example computer executable instructions stored ina tangible computer-readable storage medium such as RAM, hard disk oroptical or magnetic media. Acoustic transmitting device 104, mobiledevice 106, and remote processing server 108 can each correspond to oneor more different programs, files, circuits, or sets of instructions.Likewise, two or more instances of acoustic transmitting device 104,mobile device 106, and remote processing server 108 can be combined intoa single program, file, circuit, or set of instructions.

Instructions 406 can further include instructions for implementing abrowser, for running a specialized application, or for performing otherfunctions on mobile device 106. For instance, the specializedapplication can be used to exchange data with remote processing server108 over the network 420. Instructions 406 can includeclient-device-readable code for providing and implementing aspects ofthe present disclosure. For example, instructions 406 can includeinstructions for implementing an application associated with locationdetermination system 100 or a third party application implementing assettracking or other services on mobile device 106.

Mobile device 106 can also include data 408 that one or more processors402 retrieves, manipulates, creates, or stores. Data 408 can include,for instance, identifiers, sequences of codes, random numbers, acousticmodel data, sensor data, and/or other data. Mobile device 106 caninclude various input/output devices for providing and receivinginformation from a user, such as a touch screen, touch pad, data entrykeys, speakers, and/or a microphone suitable for voice recognition. Forinstance, mobile device 106 can multiple input buttons signifyingdifferent events. In an exemplary embodiment, a person in a hospitalcould press a button to signal distress.

Mobile device 106 can further include receiver 410. Receiver 410 can beany device or circuitry for receiving, listening for, decoding,interpreting or otherwise processing modulated acoustic signals 110 fromacoustic transmitting device 104. Mobile device 106 can also include anetwork interface used to communicate with remote processing server 108or acoustic transmitting device 104 over network 420. The networkinterface can include any suitable components for interfacing with onemore networks, including for example, transmitters, receivers, ports,controllers, antennas, or other suitable components. Mobile device 106can further include a communication system used to communicate withacoustic transmitting device 104. The communication system can include,for instance, one or more transducers (e.g. microphone devices)configured to receive acoustic (e.g. ultrasonic) signals from acoustictransmitting device 104.

In some implementations, mobile device 106 can be in communication witha remote computing device, such as remote processing server 108 overnetwork 420. Remote processing server 108 can include one or morecomputing devices. Remote processing server 108 can include one or morecomputing devices and can be implemented, for instance, as a parallel ordistributed computing system. In particular, multiple computing devicescan act together as a single remote processing server 108.

Similar to mobile device 106, remote processing server 108 can includeone or more processors 412 and memory 414. One or more processors 412can include one or more central processing units (CPUs) and/or otherprocessing devices. Memory 414 can include one or more computer-readablemedia and store information accessible by one or more processors 412,including instructions 416 that can be executed by one or moreprocessors 412 and data 418.

Data 418 can be stored in one or more databases. The data can includeidentifier information, acoustic model data, and other data required bylocation determination system 100. One or more databases can beconnected to remote processing server 108 by a high bandwidth LAN or WANor can also be connected to remote processing server 108 through network420. The one or more databases can be split up and reside in distributedor multiple locales.

Remote processing server 108 can also include a network interface usedto communicate with mobile device 106 and acoustic transmitting device104 over network 420. The network interface can include any suitablecomponents for interfacing with one more networks, including forexample, transmitters, receivers, ports, controllers, antennas, or othersuitable components.

Network 420 can be any type of communications network, such as a localarea network (e.g. intranet), wide area network (e.g. Internet),cellular network, or some combination thereof. Network 420 can alsoinclude direct connections between acoustic transmitting device 104,mobile device 106, and remote processing server 108. Network 420 caninclude any number of wired or wireless links and instrumented using anysuitable communication protocol.

Location system 400 can further include one or more instances ofacoustic transmitting device 104. Acoustic transmitting device 104 cantransmit acoustic signals (e.g. ultrasonic signals) as described inFIG. 1. In some implementations, acoustic transmitting device 104 cantransmit other suitable signals, such as radio frequency signals.Acoustic transmitting device 104 can be implemented using any suitablecomputing devices. Acoustic transmitting device 104 can include one ormore transducers configured to emit acoustic or other suitable signalsthat mobile device 106 uses to derive a location. Although FIG. 4depicts only one acoustic transmitting device 104 and mobile device 106it will be appreciated by those skilled in the art that any suitablenumber of these devices can be included in location system 400.

It is to be appreciated that the Detailed Description section, and notthe Summary and Abstract sections, should be used to interpret theclaims. The Summary and Abstract sections may set forth one or more butnot all exemplary embodiments of the present invention, and thus, shouldnot limit the present invention and the appended claims in any way.

The invention has been described above with the aid of functionalbuilding blocks illustrating the implementation of specified functionsand relationships thereof. The boundaries of these functional buildingblocks have been arbitrarily defined herein for the convenience of thedescription. Alternate boundaries may be defined so long as thespecified functions and relationships thereof are appropriatelyperformed.

It will be apparent to those skilled in the relevant art(s) that variouschanges in form and detail can be made therein without departing fromthe spirit and scope of the disclosure. Thus, the invention should notbe limited by any of the above-described exemplary embodiments, butshould be defined only in accordance with the following claims and theirequivalents.

What is claimed is:
 1. A computer-implemented method, comprising:receiving, during a time period, an acoustic signal by a mobile devicein a real-time location system in an environment, the acoustic signalhaving been transmitted by an acoustic transmitting device located inthe environment; detecting, by the mobile device, an identifier from thereceived acoustic signal, wherein the detected identifier is a member ofa rotating set of identifiers for the acoustic transmitting device; anddetermining a location of the mobile device based on the detectedidentifier and the time period.
 2. The method of claim 1, wherein theacoustic signal is an ultrasonic signal.
 3. The method of claim 1,wherein the determining the location of the mobile device is furtherbased on the detected identifier, the time period and an RF access pointto which the located tag is communicatively coupled.
 4. The method ofclaim 1, wherein the environment is a hospital.
 5. The method of claim1, wherein determining a device location of the mobile device comprises:comparing the detected identifier with pre-determined identifiers thatare valid for the time period, the pre-determined identifiers beingstored within a predefined table, and wherein an identifier of thepre-determined identifiers that matches the detected identifier isassociated with the acoustic transmitting device.
 6. The method of claim1, further comprising: transmitting, by the mobile device, the detectedidentifier of the acoustic transmitting device, via a wireless localarea network to a central server in the real-time location system.
 7. Acomputer-implemented method, comprising: determining a rotating set ofidentifiers associated with an acoustic transmitting device for areal-time location system in an environment, wherein each of theidentifiers has a respective time period of validity for associationwith the acoustic transmitting device; modulating, by the acoustictransmitting device, an acoustic signal with one identifier from therotating set of identifiers to output a modulated acoustic signal,wherein the one identifier is associated with the acoustic transmittingdevice during the respective time period of validity; and transmitting,by the acoustic transmitting device, the modulated acoustic signal to amobile device located within the environment.
 8. The method of claim 7,wherein the modulated acoustic signal is an ultrasonic signal.
 9. Themethod of claim 7, wherein each respective time period of validity foreach identifier is within a range of 3 seconds and 300 seconds.
 10. Themethod of claim 7, wherein the environment is a hospital.
 11. The methodof claim 7, wherein determining the rotating set of identifierscomprises: generating the rotating set of identifiers using apseudo-random number generator and a seed value.
 12. The method of claim7, wherein determining the rotating set of identifiers comprises:selecting the rotating set of identifiers from a pre-defined table ofidentifiers that associates the selected rotating set of identifierswith the acoustic transmitting device and respective time periods ofvalidity.
 13. A computer-implemented system, comprising: an acoustictransmitting device, configured to: determine a rotating set ofidentifiers associated for a real-time location system in anenvironment, wherein each of the identifiers has a respective timeperiod of validity for association with the acoustic transmittingdevice; modulate, by the acoustic transmitting device, an acousticsignal with one identifier from the rotating set of identifiers tooutput a modulated acoustic signal and transmit the modulated acousticsignal to a mobile device, wherein the one identifier is associated withthe acoustic transmitting device during the respective time period ofvalidity; and the mobile device, configured to: receive, during therespective time period of validity, the modulated acoustic signal;detect, a detected identifier from the received modulated acousticsignal; and determine a location of the mobile device based on thedetected identifier and the time period.
 14. The system of claim 13,wherein the modulated acoustic signal is an ultrasonic signal.
 15. Thesystem of claim 13, wherein the determining the location of the mobiledevice is further based on the detected identifier, the respective timeperiod of validity and an RF access point to which the located tag iscommunicatively coupled.
 16. The system of claim 13, wherein theacoustic transmitting device is further configured to: determine therotating set of identifiers using a pseudo-random number generator and aseed value.
 17. The system of claim 13, wherein the acoustictransmitting device is further configured to: determine the rotating setof identifiers using a predefined table that associates each identifierof the rotating set of identifiers with the acoustic transmitting devicefor the respective time period of validity.
 18. The system of claim 13,wherein the mobile device is further configured to: compare the detectedidentifier with identifiers stored within a predefined table, thepredefined table associating one of the identifiers with the acoustictransmitting device for the respective time period of validity.
 19. Thesystem of claim 13, wherein the mobile device is further configured to:transmit the location of the mobile device via a wireless local areanetwork to a central server in the real-time location system.
 20. Amobile device, comprising: an acoustic receiver configured to receive amodulated acoustic signal, wherein the modulated acoustic signalincludes an identifier, the identifier having an association with anacoustic transmitting device for a time period of validity and theidentifier having no association with the acoustic transmitting outsidethe time period of validity; and a processor configured to: detect theidentifier from the received modulated acoustic signal; and determine alocation of the mobile device based on the identifier and the timeperiod of validity.